Burn tunnel dimensions for a 8inch

Hi great people!

Thanks for a super great forum/website - and for letting me in.
I have read and seen a lot of info here and elsewhere on the net. Now i'm finally building my RMH and am hoping to get some help here. Hope you can help me get started by commenting on these couple of questions:

1: Can someone tell me a good hight and width for my burn tunnel in a 8 inch system?

2: I have made a foundation of concrete on the sand below my house. On this i'm planning to put a 4inch leca stone as a layer on top of the concrete to insulate the mass from the concrete. Is that sufficient isolation there?
Building the heat extraction tunnel of regular old used bricks. Do i need further insulation under this tunnel, or can the bottoms brick be on the leca stone?
I'm buying a vermiculite board to put under the combustion part between leca stone and firebricks.

Hope you can help me solve this out.

Thanks
Michael, Denmark

Have you read the standard book on the subject, Rocket Mass Heaters (third edition) by Ianto Evans and Leslie Jackson?
http://www.rocketstoves.com/
It has that information and so much more. For an 8" system, a feed tube and burn tunnel 7" square or maybe 7" wide x 7 1/2" high is the standard recommendation. The heat riser should be the same dimensions if built of firebrick, or 8" diameter if round.

Thanks Glenn

I have the book and have read it ... reading again right now. I just get nervous and want to make sure i don't make a brainfart, since i'm building it all alone.... nobody to catch them
Ok - i thought it needed to be even taller than wide. But almost the same i see... It's big compared to the 6inch test build i made earlier. These dimensions do not make it quite as easy with my fire bricks...
So this big heat riser will take a bit off the insulation around it between the oil drum and the heat riser... will this be fine still?
Thanks for your advise.

Michael

Hi Michael, by Leca, you mean thoses expanded clay balls? If yes, i don't see it being a problem. With the air pockets etc. Only thing i would worry about is their compresive strength.

https://en.wikipedia.org/wiki/Expanded_clay_aggregate

Hi Satamax Antone

Thanks - those leca pellets are molded into concrete made into stones. These: http://www.fc-beton.dk/lecablokke-600-10x19x49_114r78.html

Not very much insulating these stones, but thought it won't be totally wasted in the concrete mass below... I don't know what else to put under this big mass - insulating yet strong...

For compressive loads like this, perlite can work well. I know it's not "permie" but i quite like air entrained concrete for this type of aplication too.

Would you mix perlite into sand and cement - or just clay and perlite to insulate the heat riser and burn tunnel?

No cement in the mix, exept if you use "ciment fondu" . Remember to make thoses parts with expansion joints, so they can move about when hot.

For insulation, sand will not be beneficial. Just perlite and clay will do best. You want only enough clay to hold the perlite together, no more, for best insulation. I like to put the perlite in the tub first, wet it a bit by sprinkling or spraying water, then dust dry powdered clay on it and mix. Repeat until you have all the grains of perlite coated with clay and it holds together when you squeeze a ball of it, then pops apart with firm finger pressure.